virtualization - Carnegie Mellon School of Computer...

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VirtualizationRoger Dannenberg and

Dave Eckhardtbased on material from:

Mike KasickGlenn Willen

Mike CuiNovember 24, 2008

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Outline

● Introduction

● Virtualization

● x86 Virtualization

● Alternatives for Isolation

● Alternatives for “running two OSes on same machine”

● Summary

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What is Virtualization?● Virtualization:

− Process of presenting and partitioning computingresources in a logical way rather than partitioningaccording to physical reality

● Virtual Machine:− An execution environment (logically) identical to a

physical machine, with the ability to execute a fulloperating system

● The Process abstraction is related to virtualization: it’sat least similar to a physical machine

Process : OS :: OS : ?

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Advantages of the ProcessAbstraction

● Each process is a pseudo-machine● Processes have their own registers, address space, file

descriptors (sometimes)● Protection from other processes

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Disadvantages of the ProcessAbstraction

● Processes share the file system● Difficult to simultaneously use different versions of:

− Programs, libraries, configurations● Single machine owner:

− root is the superuser− Which “domain” does a machine belong to?

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Disadvantages of the ProcessAbstraction

● Processes share the same kernel− Kernel/OS specific software− Kernels are huge, lots of possibly buggy code

● Processes have limited degree of protection, even fromeach other− OOM (out of memory) killer (in Linux) frees memory when

all else fails

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Why Use Virtualization?

● Process abstraction at the kernel layer− Separate file system− Different machine owners

● Offers much better protection (in theory)− Secure hypervisor, fair scheduler− Interdomain DoS? Thrashing?

● Run two operating systems on the same machine!

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Why Use Virtualization?

● Huge impact on enterprise hosting− No longer have to sell whole machines− Sell machine slices− Can put competitors on the same physical hardware

Can separate instance of VM from instance of hardware● Live migration of VM from machine to machine

− No more maintenance downtime● VM replication to provide fault-tolerance

− Why bother doing it at the application level?

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Disadvantages of Virtual Machines

● Attempt to solve what really is an abstraction issuesomewhere else− Monolithic kernels− Not enough partitioning of global identifiers

● pids, uids, etc● Provides some interesting mechanisms, but may not

directly solve “the problem”

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Disadvantages of Virtual Machines

● Feasibility issues− Hardware support? OS support?− Admin support?− VMware ESX seems to be doing the job well

● Performance issues− Is a 10-20% performance hit tolerable?− Can your NIC or disk keep up with the load?

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Outline

● Introduction

● Virtualization




● Summary

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Full Virtualization

● IBM CP-40 (later CP/CMS & VM/CMS) (1967)− Supported 14 simultaneous S/360 virtual machines.

● Popek & Goldberg: Formal Requirements forVirtualizable Third Generation Architectures (1974)− Defines characteristics of a Virtual Machine Monitor− Describes a set of architecture features sufficient to

support virtualization

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Virtual Machine Monitor

● Equivalence:− Provides an environment essentially identical with the

original machine● Efficiency:

− Programs running under a VMM should exhibit only minordecreases in speed

● Resource Control:− VMM is in complete control of system resources

Process : Kernel :: VM : VMM

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Popek & Goldberg InstructionClassification

● Privileged instructions:− Trap if the processor is in user mode− Do not trap if in supervisor mode

● Sensitive instructions:− Attempt to change configuration of system resources− Illustrate different behaviors depending on system

configuration

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Popek & Goldberg Theorem

“... a virtual machine monitor may be constructed if theset of sensitive instructions for that computer is asubset of the set of privileged instructions.”

● All instructions must either:− Exhibit the same result in user and supervisor modes− Or, they must trap if executed in user mode

● Architectures that meet this requirement:− IBM S/370, Motorola 68010+, PowerPC, others.

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Outline

● Introduction

● Virtualization




● Summary

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x86 Virtualization

● x86 ISA does not meet the Popek & Goldbergrequirements for virtualization

● ISA contains 17+ sensitive, unprivileged instructions:− SGDT, SIDT, SLDT, SMSW, PUSHF, POPF, LAR, LSL,VERR, VERW, POP, PUSH, CALL, JMP, INT, RET,STR, MOV

− Most simply reveal the processor's CPL● Virtualization is still possible, requires a workaround

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VMware (1998)

● Runs guest operating system in ring 3− Maintains the illusion of running the guest in ring 0

● Insensitive instructions execute as is:− addl %ecx, %eax

● Privileged instructions trap to the VMM:− cli

● Performs binary translation on guest code to workaround sensitive, unprivileged instructions:− popf ⇒ int $99

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VMware (1998)

Privileged instructions trap to the VMM:cli

actually results in:int $13 (General Protection Fault)

which gets handled:void gpf_exception(int vm_num, regs_t *regs){ switch (vmm_get_faulting_opcode(regs->eip)) { ...

case CLI_OP: vmm_defer_interrupts(vm_num);

break;...

}}

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VMware (1998)

A sensitive, unprivileged instruction:popf (restore %EFLAGS from the stack)

we would like to result in:int $13 (General Protection Fault)

but actually results in:%EFLAGS ← all bits from stack except IOPL

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VMware (1998)

So, VMware performs binary translation on guest code:popf

VMware translates to:int $99 (popf handler)

which gets handled:void popf_handler(int vm_num, regs_t *regs){ regs->eflags = *(regs->esp); regs->esp++;}

Note: technique is similar to software fault isolation (patent) -- 1993 (!)Steven Lucco’s thesis, UCB.

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Virtual Memory

● Kernels can access physical memory and implementvirtual memory.

● How do we virtualize physical memory?− Use virtual memory (obvious so far, isn’t it?)

● If guest kernel runs in virtual memory, how does itprovide virtual memory for processes?− VMM may have to “shadow” page mapping tables− Set CR3 traps, constructs real virtual memory− Writes to page directories and page tables are trapped,

mapped to “shadow” tables

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Hardware Assisted Virtualization

● Recent variants of the x86 ISA that meet Popek &Goldberg requirements− Intel VT-x (2005), AMD-V (2006)

● VT-x introduces two new operating modes:− VMX root operation & VMX non-root operation− VMM runs in VMX root, guest OS runs in non-root− Both modes support all privilege rings− Guest OS runs in (non-root) ring 0, no illusions necessary

● At present, binary translation as used in VMware isfaster than hardware solution.

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Paravirtualization(Denali 2002, Xen 2003)

● First observation:− If OS is open, then it can be modified at the source level to

supported limited virtualization● Paravirtualizing VMMs (hypervisors) virtualize only a subset of the

x86 execution environment● Run guest OS in rings 1-3

− No illusion about running in a virtual environment− Guests may not use sensitive, unprivileged instructions and

expect a privileged result● Requires source modification only to guest kernels● No modifications to user level code and applications

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Paravirtualization(Denali 2002, Xen 2003)

● Second observation:− Regular VMMs must emulate hardware for devices

● Disk, ethernet, etc● Performance is poor due to constrained device API● Emulated hardware, x86 ISA, inb/outb, PICs

− Already modifying guest kernel, why not provide virtualdevice drivers?

● Faster API?− Hypercall interface:

syscall : kernel :: hypercall : hypervisor

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VMware vs. Paravirtualization

● Kernel's device communication with VMware (emulated):

void nic_write_buffer(char *buf, int size){ for (; size > 0; size--) {

nic_poll_ready(); outb(NIC_TX_BUF, *buf++); }

}

● Kernel's device communication with hypervisor (hypercall):

void nic_write_buffer(char *buf, int size){ vmm_write(NIC_TX_BUF, buf, size);}

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Xen (2003)

● Popular hypervisor supporting paravirtualization− Hypervisor runs on hardware− Runs two kinds of kernels− Host kernel runs in domain 0 (dom0)

● Required by Xen to boot● Hypervisor contains no peripheral device drivers● dom0 needed to communicate with devices● Supports all peripherals that Linux or NetBSD do!

− Guest kernels run in unprivileged domains (domUs)

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Xen (2003)● Provides virtual devices to guest kernels

− Virtual block device, virtual ethernet device− Devices communicate with hypercalls & ring buffers− Can also assign PCI devices to specific domUs

● Video card● Also supports hardware assisted virtualization (HVM)

− Allows Xen to run unmodified domUs− Useful for bootstrapping− Also used for “the OS” that can't be source modified

● Supports Linux & NetBSD as dom0 kernels

● Linux, FreeBSD, NetBSD, and Solaris as domUs

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Outline

● Introduction

● Virtualization




● Summary

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chroot

● Runs a Unix process with a different root directory− Almost like having a separate file system

● Share the same kernel & non-filesystem “things”− Networking, process control

● Only a minimal sandbox.− /proc, /sys− Resources: I/O bandwidth, cpu time, memory,

disk space, …

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User-mode Linux

● Runs a guest Linux kernel as a user space processunder a regular Linux kernel

● Requires highly modified Linux kernel● No modification to application code● Used to be popular among hosting providers● More mature than Xen, roughly equivalent, but much

slower because Xen is designed to host kernels

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Container-based OS Virtualization

● Allows multiple instances of an OS to run in isolated containersunder the same kernel

● Assumptions:− Want strong separation between “virtual machines”− But we can trust the kernel− Every “virtual machine” can use the same kernel version

● It follows that:− Don’t need to virtualize the kernel− Instead, beef up naming and partitioning inside the kernel: Each

container can have:● User id, pid, tid space● Domain name● Isolated file system, OS version, libraries, etc.

● Total isolation between containers without virtualization overhead.● VServer, FBSD Jails, OpenVZ, Solaris Containers

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Outline

● Introduction

● Virtualization




● Summary

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Full System Simulation(Simics 1998)

● Software simulates hardware components that make upa target machine

● Interpreter executes each instruction & updates thesoftware representation of the hardware state

● Approach is very accurate but very slow● Great for OS development & debugging● Break on triple fault is better than a reset

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System Emulation(Bochs, DOSBox, QEMU)

● Seeks to emulate just enough of system hardware components tocreate an accurate “user experience”

● Typically CPU & memory subsystems are emulated− Buses are not− Devices communicate with CPU & memory directly

● Many shortcuts taken to achieve better performance− Reduces overall system accuracy− Code designed to run correctly on real hardware executes

“pretty well”− Code not designed to run correctly on real hardware exhibits

wildly divergent behavior● E.g. run legacy 680x0 code on PowerPC, run Windows on ??

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System Emulation Techniques

● Pure interpretation:− Interpret each guest instruction− Perform a semantically equivalent operation on host

● Static translation:− Translate each guest instruction to host once− Happens at startup− Limited applicability, no self-modifying code

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System Emulation Techniques

● Dynamic translation:− Translate a block of guest instructions to host

instructions just prior to execution of that block− Cache translated blocks for better performance

● Dynamic recompilation & adaptive optimization:− Discover what algorithm the guest code implements− Substitute with an optimized version on the host− Hard

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QEMU (2005)

● Open source fast processor/machine emulator● Run an i386, amd64, arm, sparc, powerpc, or mips OS

on your i386, amd64, powerpc, alpha, sparc, arm, ors390 computer

● Can run any i386 (or other) OS as a user application− Complete with graphics, sound, and network support− Don't even need to be root!

● Tolerable performance for real world Oses− Orders of magnitude faster than Simics

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QEMU's Portable Dynamic Translator

● Cute hack: uses GCC to pregenerate translated code● Code executing on host is generated by GCC

− Not hand written● Makes QEMU easily portable to architectures that GCC

supports− “The overall porting complexity of QEMU is estimated to

be the same as the one of a dynamic linker.”

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Instructions for a given architecture are divided intomicro-operations. For example:

addl $42, %eax # eax += 42divides into:

movl_T0_EAX # T0 = eaxaddl_T0_im # T0 += 42movl_EAX_T0 # eax = T0

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● At (QEMU) compile time, each micro-op is compiledfrom C into an object file for the host architecture− dyngen copies the machine code from object files− Object code used as input data for code generator

● At runtime, code generator reads a stream of micro-opsand emits a stream of machine code− By convention, code executes properly as emitted

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Micro-operations are coded as individual C functions:void OPPROTO op_movl_T0_EAX(void) { T0 = EAX }void OPPROTO op_addl_T0_im(void) { T0 += PARAM1 }void OPPROTO op_movl_EAX_T0(void) { EAX = T0 }

which are compiled by GCC to machine code:op_movl_T0_EAX:

movl 0(%ebp), %ebxret

op_addl_T0_im:addl $42, %ebxret

op_movl_EAX_T0:movl %ebx, 0(%ebp)ret

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dyngen strips away function prologue and epilogue:op_movl_T0_EAX:

movl 0(%ebp), %ebx

op_addl_T0_im:addl $42, %ebx

op_movl_EAX_T0:movl %ebx, 0(%ebp)

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At runtime, QEMU translate the instruction:add $42, %eax

into the micro-op sequence:op_movl_T0_EAXop_addl_T0_imop_movl_EAX_T0

and then into machine code:movl 0(%ebp), %ebxaddl $42, %ebxmovl %ebx, 0(%ebp)

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● When QEMU encounters untranslated code, it translateseach instruction until the next branch− Forms a single translation block

● After each code block is executed, the next block islocated in the block hash table− Indexed by CPU state− Or, block is translated if not found

● Write protects guest code pages after translation− Write attempt indicates self modifying code− Translations are invalidated on write attempt

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Outline

● Introduction

● Virtualization




● Summary

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Summary

● Virtualization is big in enterprise hosting● {Full, hardware assisted, para-}virtualization● Containers: VM-like abstraction with high efficiency● Emulation is a slower alternative, more flexibility

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Further Reading● Gerald J. Popek and Robert P. Goldberg.

Formal requirements for virtualizable third generation architectures.Communications of the ACM, 17(7):412-421, July 1974.

● John Scott Robin and Cynthia E. Irvine.Analysis of the intel pentium’s ability to support a secure virtual machine monitor.In Proceedings of the 9th USENIX Security Symposium, Denver, CO, August 2000.

● Gil Neiger, Amy Santoni, Felix Leung, Dion Rodgers, and Rich Uhlig.Intel Virtualization Technology: Hardware support for efficient processor virtualization.Intel Technology Journal, 10(3):167-177, August 2006.

● Paul Barham, Boris Dragovic, Keir Fraser, Steven Hand, Tim Harris, Alex Ho, Rolf Neugebauer,Ian Pratt, and Andrew Warfield.Xen and the art of virtualization.In Proceedings of the 19th ACM Symposium on Operating Systems Principles, pages 164-177,Bolton Landing, NY, October 2003.

● Yaozu Dong, Shaofan Li, Asit Mallick, Jun Nakajima, Kun Tian, Xuefei Xu, Fred Yang, andWilfred Yu. Extending Xen with Intel Virtualization Technology.Intel Technology Journal, 10(3):193-203, August 2006.

● Stephen Soltesz, Herbert Potzl, Marc E. Fiuczynski, Andy Bavier, and Larry Peterson.Container-based operating system virtualization: A scalable, high-performance alternative tohypervisors.In Proceedings of the 2007 EuroSys conference, Lisbon, Portugal, March 2007.

● Fabrice Bellard.QEMU, a fast and por table dynamic translator.In Proceedings of the 2005 USENIX Annual Technical Conference, Anaheim, CA, April 2005.

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